The chemical industry performs numerous equilibrium controlled reactions to manufacture a wide range of chemical raw materials, intermediates and products. Product yield obtained in such equilibrium controlled reactions is typically limited by the thermodynamic equilibrium of the reaction. Therefore, such reactions are typically operated at an elevated temperature for endothermic reactions or at a reduced temperature for exothermic reactions in order to shift equilibrium toward the product direction. Thus, the chemical industry has been searching for improved processes for operating equilibrium controlled reactions at reduced temperatures for endothermic reactions wherein product yield is not substantially diminished due to unfavorable thermodynamic equilibrium constants.
Representative equilibrium controlled reactions include methane and hydrocarbon steam reforming reactions which are used to manufacture hydrogen or synthesis gas, the water gas shift reaction for converting CO to CO.sub.2, as well as the reverse water gas shift reaction for converting CO.sub.2 to CO. Some of these reactions are typically carried out at relatively high temperatures to shift the equilibrium toward the product direction as well as to obtain relatively faster reaction kinetics. Significant efforts have been described in the literature to improve reaction kinetics by identifying new catalysts and by controlling process operating conditions. Additionally, the concept of removing a product from a reaction zone to increase product conversion is well known.
Representative processes for operating equilibrium controlled reactions include an article by Vaporciyan and Kadlec (AlChE Journal, Vol. 33, No. 8, August 1987) which discloses a unit operation comprising a rapid pressure swing cycle in a catalytic-adsorbent bed to effect both continuous gas-phase reaction and separation. The hybrid device combines features of a pressure swing adsorber with those of a flow-forced catalytic reactor.
Westerterp and coworkers (Hydrocarbon Processing) p. 69 (November 1988) disclose two process schemes for improving conversion of hydrogen and carbon monoxide to methanol. The first embodiment employs a Gas-Solid-Solid Trickle Flow Reactor (GSSTFR) wherein a solid adsorbent is trickled through a packed bed reactor to remove methanol from the reaction zone which results in increased production of methanol. The adsorbent saturated with methanol is collected on a continuous basis using multiple storage tanks wherein the methanol is desorbed by reducing the pressure. The second embodiment employs a Reactor System with Interstage Product Removal (RSIPR) wherein methanol is synthesized in several stages and removed utilizing a liquid solvent. High conversion of methanol per pass is achieved in a series of adiabatic or isothermal fixed bed reactors. Product is selectively removed in absorbers situated between the respective reactor stages.
Prior art processes for conducting simultaneous reaction and adsorption steps have not achieved commercial success because product flow rates do not remain sufficiently constant and the desired products are present in unacceptably low concentrations with respect to the undesired reaction products, unreacted feedstock and purge fluids. Industry is searching for a process for operating equilibrium controlled reactions which can be operated in continuous mode at reduced reaction temperatures wherein a reaction product can be produced in substantially pure form at high conversion, under relatively constant flow rate and at feedstock pressure.